Now slang supports "-I" options.

Change-Id: I32aef6c913d9d30f9982a593397145f1562aecf9
diff --git a/core/definitions.mk b/core/definitions.mk
index 26ab512..a5708ae 100644
--- a/core/definitions.mk
+++ b/core/definitions.mk
@@ -758,6 +758,7 @@
   --allow-rs-prefix \
   -o $(PRIVATE_RS_OUTPUT_DIR)/res/raw \
   -p $(PRIVATE_RS_OUTPUT_DIR)/src \
+  $(foreach inc,$(PRIVATE_RS_INCLUDES),$(addprefix -I , $(inc))) \
   $(PRIVATE_RS_SOURCE_FILES)
 $(hide) mkdir -p $(dir $@)
 $(hide) touch $@